The specification of the French published patent application FR 2 593 752A discloses a system which comprises a fuel tank having a filling pipe, a first conduit referred to as an overflow duct, a second conduit referred to as a gas vent duct, and a third conduit referred to as a fresh air duct. The filling pipe terminates in an orifice with a fuel cap for covering the orifice so as to close the pipe when not in use for filling purposes.
The overflow duct extends from the interior of the fuel tank, and terminates at the level of the upper part of the filling pipe, and its purpose is to evacuate any excess fuel from the fuel tank towards the filling pipe. The gas vent duct extends from the top of the fuel tank, and again terminates at the level of the upper part of the filling, pipe. The gas vent duct is required in order to permit all the necessary variations to take place in the volume of residual air present in the upper part of the tank; it also serves to accommodate any possible surges of fuel. The gas vent duct is able to be in communication with the filling pipe after the filling orifice has been closed with the fuel cap.
The fresh air duct is the external venting means whereby any build up of fuel vapour in the system can be allowed to escape to the atmosphere outside the vehicle. This may typically occur for example during refuelling. By the same token there may be an excess of pressure (over-pressure) in the fuel tank or in the system, due to increase in the volume of fuel in the tank and the consequent presence of fuel vapours and reduction in the volume within the tank available for accommodation of air in the latter. Thus the purpose of the fresh air duct is either to vent the tank so as to allow vapours and over-pressures of air to escape, or to correct any under-pressure resulting from consumption of the fuel while the vehicle is running, by admitting free air from outside into the tank through the fresh air duct itself.
The specification of European published patent application No. EP 0 336 788A discloses the provision of a safety valve between the gas vent duct and the fresh air vent, with a view to preventing any fuel from escaping from the fuel tank. Such escape of fuel may be caused by jolting in the vehicle or by the positioning of the vehicle on a steep slope (for example when stationary); or even by the vehicle being turned upside down in a accident.
This safety valve comprises a valve member which is adapted to close an outlet, by means of either a float which is actuated by the fuel itself, or a biassing means which is responsive to the inclination of the vehicle or to accelerations or decelerations of the vehicle. Such a biassing means may for example comprise either a ball which is mounted between inclined ramps, or a calibrated spring.
As described in the above mentioned European patent specification, the safety valve may be used in association with a fourth duct, referred to as a scavenging (or de-pollution) duct. Such a scavenging duct is described more fully in the specification of U.S. Pat. No. 4,742,809, and serves to separate the air from the fuel vapours by condensing the latter so that they can then return in liquid form to the fuel tank: the scavenging duct is however so arranged that clean air can still escape to the outside.
The present Applicant has found however that these various known arrangements have a number of drawbacks. In particular, in the situation in which the valve member of the safety valve is immobilised in a position in which it obstructs or closes the outlet orifice of the safety valve, any excess or deficiency in pressure that may arise is unable to be corrected, and any fuel vapour that may be present in the interior of the fuel tank is unable to be evacuated, by passage of air into the system through the said outlet orifice, or by passage of air and/or vapour from the interior to the outside via the same orifice, as the case may be.
In addition, if the scavenging duct is obstructed by some impurity, the fuel vapours are unable to be conveyed to the scavenging system, and are therefore liable to set up an excess of pressure in the fuel tank which is then unable to be corrected.